The present invention relates to an error correction device suitable for a communication system using a concatenated code.
The Unexamined Japanese Patent Application Publication No. Hei 7-22968 discloses a conventional communication system using a concatenated code and constituted by an outside encoder 11 and an inside encoder 12 which are provided on the transmission side, and an inside decoder 13 and an outside decoder 14 which are provided on the reception side, as shown in FIG. 8. Viterbi algorithm using maximum-likehood decoding is applied to decoding of an internal code to calculate the reliability of a series of decoded data. As the means for calculating the reliability of a series of decoded data, SOVA (Soft Output Viterbi Algorithm) is used for obtaining reliability information for every symbol as shown in FIG. 9, as disclosed in the Unexamined Japanese Patent Application Publication No. Hei 8-251144. On the other hand, as an example of the external code, a Reed Solomon code is used as disclosed in the Unexamined Japanese Patent Application Publication No. Hei 6-205054.
In non-speech communication such as data transmission, etc., high transmission quality with a lower bit error ratio (thereinafter abbreviated to "BER") is required in comparison with speech communication. In addition, in mobile radio communication or the like, it is desired to keep up the life of a battery of a portable terminal for a long time, and it is therefore required to reduce the output of transmission to thereby reduce the power consumption. If the BER can be made lower, a required Eb/Io can be designed to be smaller so that transmission can be performed with a lower output to thereby reduce the power consumption of the portable terminal.
However, conventional communication devices using a concatenated code were designed to attain less delay rather than low BER because they were directed mainly to be used for speech communication. Therefore, there was a problem that they cannot be regarded as optimum systems for non-speech communication. Because of the problem, it was inevitable to increase the transmission power, and if the transmission power was increased, there occurred another problem that the increased transmission power caused interference with transmission signals from other terminals.
In addition, when a concatenated code was used, maximum-likehood decoding based on Viterbi algorithm was performed for decoding an internal code, and the reliability of a series of decoded data was obtained from maximum-likehood information. Therefore, when an external code was intended to be decoded by soft judgement, the soft judgement decoding could not help using only the series of decoded data of the internal code and the maximum-likehood information thereof as input signals, so that there was a problem that error correction could not be attained with a high accuracy.